This section provides background information related to the present disclosure which is not necessarily prior art.
Thermal imaging techniques are used frequently to measure the temperature of hot objects, especially in situations where the object's temperature fluctuates rapidly or is too high for more straightforward, thermocouple-based methods. Thermal imaging, sometimes referred to pyrometry, relies on the inherent black body emission of hot objects, and depending on the wavelengths being monitored is often used to monitor temperatures between 500 K and 3000 K. Pyrometry is often used as a process monitoring technique in many industrial applications including additive manufacturing, metal processing such as refining, casting, and smelting, drilling, and related processes.
One common challenge associated with pyrometry is calibration over the appropriate temperature range. The most straightforward approach is to use a black body light source, usually a resistive furnace with a small aperture that allows the thermal emission to escape the black body cavity. Such sources produce a near-ideal black body spectrum over the entire electromagnetic spectrum. As such, they are ideal calibrants for thermal imagers or point pyrometers. However, such black body sources that operate above 1500° C. are very large, often up to 2 meters tall. Such black body sources are also time consuming to use, often taking a few hours to reach the intended temperature. For these reasons, they are not desirable and/or practical for most applications.
Another common solution for providing a black body spectrum is a well calibrated incandescent lamp. The amount of current supplied to the lamp is varied, and the temperature (and consequently the black body spectrum) of the lamp also changes. However, production of such specialty lamps is difficult and therefore suppliers are rare. Furthermore, these lamps do not produce a perfect black body spectrum due to absorptivity by glass components, changing spectral properties with age, and other inconsistencies.
The previously mentioned black body calibrant sources all rely on the generation of a true black body spectrum. An alternate approach is to produce a synthetic approximation of the black body spectrum using a broadband white light source and assorted filters. This is conceptually similar to the approach used by many solar simulator devices, which are primarily used to test the performance of photovoltaic cells. In the invention we claim here, we use multiple light sources and filters to generate a synthetic black body spectrum over two set wavelength ranges, suitable for calibrating a two-band thermal camera or two color pyrometer.